1. Field of the Invention
The present invention relates generally to an electronic test set for measuring the IR free potential of a reference electrode with an impressed output applied. In particular, measured cell potential and/or current is continuously monitored using a micro-processor or logic circuit based, real-time data compilation means, and is automatically regulated using an auto-adjusting DC supply regulation means. Reference cell potential measurements free of distorting effects, residual components, and noise are periodically logged using communications boards, and the system can be remotely monitored and configured via telephone or over a network such as the World Wide Web (WWW).
2. Description of the Related Art
Cathodic protection impressed current is employed to prevent corrosion of metallic structures. Cathodic protection systems maintain a voltage potential between the structure and a ground reference cell. The National Association of Corrosion Engineers (NACE) has established, and industry has adopted, criteria with respect to measuring reference cell potential. The criteria provides for regulation of the upper and lower maximum allowable cell potentials for given applications. Specifically, industry has adopted a cell potential range of 850 millivolts to 1.2 volts DC for use in the protection of underground storage tanks and pipelines.
Maintenance of the correct impressed currents is very important. Inadequate cathodic protection may result from the cell potential falling below 850 millivolts. Conversely, cell potential above 1.2 volts may result in protective coatings separating from the structure.
Measuring reference cell potential and adjusting impressed current output requires the expertise of trained personnel, which is, in general, beyond the capabilities of many end users. This is partly due to the use of measurement systems employing the most economical means. A common technique used involves shutting off the impressed current and measuring cell potential as soon as possible thereafter. Known in the art then, this method requires a given amount of skill and relies upon the operator correctly interpreting measured results.
Traditionally, cathodic protection of structures is practiced through the use of impressed current. Occasionally an interface for controlling the current is provided between a rectifier and an external communications link. As an example, interfacing rectifiers to GPS units in order to perform a synchronous shutdown so that manual readings may be taken along a pipeline is practiced. See U.S. Pat. No. 5,785,842 (Speck), for example.
However, even a slight misinterpretation may result in a correspondingly incorrect adjustment of impressed current output. The need for interpretation of measured cell potential is due to cell potential decay, which begins immediately after impressed current is shut down. The rate of cell potential decay is a function of structure size and tank field polarization. In general, a small tank field will experience a more rapid decay in cell potential as opposed to a larger tank field.
As an example of error being induced in this manner, a technician taking measurements of cell potential just after impressed current shutdown may attempt to memorize the falling voltage reading approximately one to two seconds after shutdown. Because the decay is time-dependent, any deviation from a strict standard for interpretation compromises accuracy for the sake of economy.
One of the features of the present invention is to provide the end user with a continuous voltage readout of IR free cell potential with impressed current applied. These readings may be observed locally and/or remotely and must be free of distorting effects, residual components, and noise. Yet another feature allows the end user the ability to manually execute or program periodic execution of an impressed current shut down at which time a timer circuit will automatically activate and take multiple cell potential readings over a period of time, and thereafter compute, store, and display IR free cell potential. The manual shutdown serves two purposes: comparing shutdown cell potential results with the impressed current applied cell potential, and comparing the two readings for accuracy and for DC supply output adjustment.
Some rectifier manufacturers use circuitry that detects an upper and lower rectifier output. This circuit is often used in an attempt to correlate rectifier output and reference cell potential. The premise for this correlation is based on an assumption that there is a given degree of linear proportionality with respect to rectifier output verses reference cell potential. Using this technique, the rectifier output is adjusted to produce an upper allowable cell potential. The circuit is then adjusted to activate a light indicator or remote alarm. The above procedure is repeated for the lower allowable cell potential. Correlating rectifier output to reference cell potential compromises accuracy and requires the skills of trained technicians which is, in general, beyond the technical capabilities of most end users.
There is a need, then, for a continuously operable cathodic protection method and system that can automatically validate readings and automatically adjust output voltage to the structure, and which can monitor both current and reference cell potential measurements on-demand with real-time data display and compilation capabilities.
The objective of the present invention is to provide a system and method implemented therein for regulating output and continuously and remotely monitoring measurements of reference cell potential and/or current. Both reference cell potential and current is controlled using a logic circuit and/or microprocessor-based hardware system, and associated software, that monitors and logs cell potential readings and allows for remote monitoring of the readings via telephone and/or the Internet and/or other network compatible hand-held components. The cathodic protection device (CPD) automatically adjusts the DC supply""s output providing the means for remotely maintaining cell potential within allowable operating limits. Yet another object of this invention is to provide an automatic electronic testing unit, which checks the proper operation of the cathodic protection impressed current system. Still a further object of this invention is to provide a system and method, which allows for a microprocessor-based shutdown at which time the CPD unit will automatically measure and store the cell potential readings, thereby maintaining a consistent shut-down interval with the measurement of the decay time being extrapolated. The stored measurements are then used for determining proper operation of both CPD and adjustable DC supply. Still another object of this invention is to provide a system that allows end users the means for the remote verification of proper operation and the means for system adjustment by personnel having minimum technical skills.
Therefore, a system and method is provided wherein a cathodic protection device operable in both current and reference cell modes, is adapted to vary output, which output is usually an adjustable DC supply. This output can correspond to either current or reference cell potential measurements that are optionally communicated to a central computer using internal interface units piggybacked on the cathodic protection devices. The voltage detection means, working in conjunction with each CPD, provides all measurements and inputs them to the central computer, wherein they are compared and cross-checked using a voting scheme means. This improves confidence in the measurements to meet target window regulation needs if the output must be varied or changed.
The system detects deviation from desired regulation occurring because of adjustable DC supply limitations, then flags and report such events. Also, the CPD uses a means of monitoring cell potential while the adjustable DC supply is operational, in which case real time data-compilation and monitoring is made possible by way of a reference cell potential monitor. This reference cell potential monitor can be logic circuit or micro-processor based. The monitor uses detection signal processing techniques to accurately derive the amplitude of the DC bias by removing identifiable components and noise such as distorting effects, signal minimum dips, and residual line frequency components. This DC bias is then correlated to the true reference cell potential measurement.
This system uses cathodic protection devices that further provide a means of measuring and calculating cell decay during an impressed current shutdown. This allows validation through a comparison of the calculated shutdown decay rate to a pre-entered estimate of structure decay rate, and then flags and reports errors. The system validates itself by using the best of a calculated shutdown decay rate or pre-entered decay rate to calculate time zero cell potential and compares it to the real time monitor value and flags and reports errors. The system can use central station software in conjunction with a user configurable web page for the purpose of making easily accessible reports available to operators and regulatory agencies concerning the operational history of an individual cathodic protection device or group of cathodic protection devices. The system can also use Internet addressing in conjunction with a telephone system, cell phone, radio links, or other Internet interfaces to access individual or groups of cathodic protection devices.
To arrive at a true reference cell potential measurement while impressed current is being applied to the structure, noise, distorting effects, and residual components such as a 50 or 60 Hz line component must be eliminated. The method generally comprises the steps of generating a sync signal and establishing a sample collection window for the sync signal so sample points of the reference cell signal are captured or acquired along minimums of the reference cell signal. As the signal is measured within this window, distorting effects such as an IR drop, and the small negative approaching dips on the leading edges of the minimums are removed. After then passing the sample points through a moving average filter, a residual component such as the 60 Hz component is removed and the DC bias is formed. This DC bias is then correlated to the true reference cell potential measurement.
The sample collection window is adjustable so at least one delay can be applied to the signal within the window at positions before and after the occurrence of the IR component and dips respectively. Using a simplified assumption, the delay may also be applied by determining a midway point on the at least one of the minimums of the reference cell signal such that there are no bounds on the window since only the time interval between falling and rising portions of the waveform is determined. The applied delays may further be calculated to account for phase lag introduced to the signal by a rectifier transformer supplying power to the structure.